RFC 5086 TDM Circuit Emulation Service over PSN December 20071. Introduction
This document describes a method for encapsulating structured (NxDS0)
Time Division Multiplexed (TDM) signals as pseudowires over packet-
switching networks (PSN). In this regard, it complements similar
work for structure-agnostic emulation of TDM bit-streams [RFC4553].
Emulation of NxDS0 circuits provides for saving PSN bandwidth, and
supports DS0-level grooming and distributed cross-connect
applications. It also enhances resilience of CE devices to effects
of loss of packets in the PSN.
The CESoPSN solution presented in this document fits the Pseudowire
Emulation Edge-to-Edge (PWE3) architecture described in [RFC3985],
satisfies the general requirements put forth in [RFC3916], and
specific requirements for structured TDM emulation put forth in
[RFC4197].
2. Terminology and Reference Models2.1. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
The terms defined in [RFC3985], Section 1.4, and in [RFC4197],
Section 3, are consistently used without additional explanations.
This document uses some terms and acronyms that are commonly used in
conjunction with TDM services. In particular:
o Loss of Signal (LOS) is a common term denoting a condition where a
valid TDM signal cannot be extracted from the physical layer of
the trunk. Actual criteria for detecting and clearing LOS are
described in [G.775].
o Frame Alignment Signal (FAS) is a common term denoting a special
periodic pattern that is used to impose TDM structures on E1 and
T1 circuits. These patterns are described in [G.704].
o Out of Frame Synchronization (OOF) is a common term denoting the
state of the receiver of a TDM signal when it failed to find valid
FAS. Actual criteria for declaring and clearing OOF are described
in [G.706]. Handling of this condition includes invalidation of
the TDM data.
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o Alarm Indication Signal (AIS) is a common term denoting a special
bit pattern in the TDM bit stream that indicates presence of an
upstream circuit outage. Actual criteria for declaring and
clearing the AIS condition in a TDM stream are defined in [G.775].
o Remote Alarm Indication (RAI) and Remote Defect Indication (RDI)
are common terms (often used as synonyms) denoting a special
pattern in the framing of a TDM service that is sent back by the
receiver that experiences an AIS condition. This condition cannot
be detected while an LOS, OOF, or AIS condition is detected.
Specific rules for encoding this pattern in the TDM framing are
discussed in [G.775].
We also use the term Interworking Function (IWF) to describe the
functional block that segments and encapsulates TDM into CESoPSN
packets and, in the reverse direction, decapsulates CESoPSN packets
and reconstitutes TDM.
2.2. Reference Models
Generic models that have been defined in Sections 4.1, 4.2, and 4.4
of [RFC3985] are fully applicable for the purposes of this document
without any modifications.
The Network Synchronization reference model and deployment scenarios
for emulation of TDM services have been described in [RFC4197],
Section 4.3.
Structured services considered in this document represent special
cases of the "Structured bit stream" payload type defined in Section3.3.4 of [RFC3985]. In each specific case, the basic service
structures that are preserved by a CESoPSN PW are explicitly
specified (see Section 3 below).
In accordance with the principle of minimum intervention ([RFC3985],
Section 3.3.5), the TDM payload is encapsulated without any changes.
2.3. Requirements and Design Constraints
The CESoPSN protocol has been designed in order to meet the following
design constraints:
1. Fixed amount of TDM data per packet: All the packets belonging to
a given CESoPSN PW MUST carry the same amount of TDM data. This
approach simplifies compensation of a lost PW packet with a
packet carrying exactly the same amount of "replacement" TDM data
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2. Fixed end-to-end delay: CESoPSN implementations SHOULD provide
the same end-to-end delay between a given pair of CEs regardless
of the bit rate of the emulated service.
3. Packetization latency range: a) All the implementations of
CESoPSN SHOULD support packetization latencies in the range 1 to
5 milliseconds. b) CESoPSN implementations that support
configurable packetization latency MUST allow configuration of
this parameter with the granularity, which is a multiple of 125
microseconds.
4. Common data path for services with and without CE application
signaling (e.g., Channel-Associated Signaling (CAS)-- see
[RFC4197]): If, in addition to TDM data, CE signaling must be
transferred between a pair of CE devices for the normal operation
of the emulated service, this signaling is passed in dedicated
signaling packets specific for the signaling protocol while
format and processing of the packets carrying TDM data remain
unchanged.
3. Emulated Services
In accordance with [RFC4197], structured services considered in this
specification are NxDS0 services, with and without CAS.
NxDS0 services are usually carried within appropriate physical
trunks, and Provider Edges (PEs) providing their emulation include
appropriate Native Service Processing (NSP) blocks, commonly referred
to as Framers.
The NSPs may also act as digital cross-connects, creating structured
TDM services from multiple synchronous trunks. As a consequence, the
service may contain more timeslots that could be carried over any
single trunk, or the timeslots may not originate from any single
trunk.
The reference PE architecture supporting these services is described
in Appendix B.
This document defines a single format for packets carrying TDM data
regardless of the need to carry CAS or any other CE application
signaling. The resulting "basic NxDS0 service" can be extended to
carry CE application signaling (e.g., CAS) using separate signaling
packets. Signaling packets MAY be carried in the same PW as the
packets carrying TDM data or in a separate dedicated PW.
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In addition, this document also defines dedicated formats for
carrying NxDS0 services with CAS in signaling sub-structures in some
of the packets. These formats effectively differ for NxDS0 services
that originated in different trunks so that their usage results in
emulating trunk-specific NxDS0 services with CAS.
4. CESoPSN Encapsulation Layer4.1. CESoPSN Packet Format
The CESoPSN header MUST contain the CESoPSN Control Word (4 bytes)
and MAY also contain a fixed RTP header [RFC3550]. If the RTP header
is included in the CESoPSN header, it MUST immediately follow the
CESoPSN control word in all cases except UDP demultiplexing, where it
MUST precede it (see Figures 1a, 1b, and 1c below).
Note: The difference in the CESoPSN packet formats for IP PSN using
UDP-based demultiplexing and the rest of the PSN and demultiplexing
combinations, is based on the following considerations:
1. Compliance with the existing header compression mechanisms for
IPv4/IPv6 PSNs with UDP demultiplexing requires placing the RTP
header immediately after the UDP header.
2. Compliance with the common PWE3 mechanisms for keeping PWs Equal
Cost Multipath (ECMP)-safe for the MPLS PSN by providing for PW-
IP packet discrimination (see [RFC3985], Section 5.4.3). This
requires placing the PWE3 control word immediately after the PW
label.
3. Commonality of the CESoPSN packet formats for MPLS networks and
IPv4/IPv6 networks with Layer 2 Tunneling Protocol Version 3
(L2TPv3) demultiplexing facilitates smooth stitching of L2TPv3-
based and MPLS-based segments of CESoPSN PWs (see [PWE3-MS]).
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UDP ports MUST be manually configured by both endpoints of the PW.
The configured destination port together with both the source and
destination IP addresses uniquely identifies the PW for the receiver.
All UDP port values that function as PW labels SHOULD be in the range
of dynamically allocated UDP port numbers (49152 through 65535).
While many UDP-based protocols are able to traverse middleboxes
without dire consequences, the use of UDP ports as PW labels makes
middlebox traversal more difficult. Hence, it is NOT RECOMMENDED to
use UDP-based PWs where port-translating middleboxes are present
between PW endpoints.
4.3. CESoPSN Control Word
The structure of the CESoPSN Control Word that MUST be used with all
combinations of the PSN and demultiplexing mechanisms described in
the previous section is shown in Figure 2 below.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0|0|0|0|L|R| M |FRG| LEN | Sequence number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2. Structure of the CESoPSN Control Word
The use of Bits 0 to 3 is described in [RFC4385]. These bits MUST be
set to zero unless they are being used to indicate the start of an
Associated Channel Header (ACH). An ACH is needed if the state of
the CESoPSN PW is being monitored using Virtual Circuit Connectivity
Verification [RFC5085].
L - if set, indicates some abnormal condition of the attachment
circuit.
M - a 2-bit modifier field. In case of L cleared, this field allows
discrimination of signaling packets and carrying RDI of the
attachment circuit across the PSN. In case of L set, only the
'00' value is currently defined; other values are reserved for
future extensions. L and M bits can be treated as a 3-bit code
point space that is described in detail in Table 1 below.
R - if set by the PSN-bound IWF, indicates that its local CE-bound
IWF is in the packet loss state, i.e., has lost a pre-configured
number of consecutive packets. The R bit MUST be cleared by the
PSN-bound IWF once its local CE-bound IWF has exited the packet
loss state, i.e., has received a pre-configured number of
consecutive packets.
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LEN (bits (10 to 15) MAY be used to carry the length of the CESoPSN
packet (defined as the size of the CESoPSN header + the payload size)
if it is less than 64 bytes, and MUST be set to zero otherwise.
Note: If fixed RTP header is used in the encapsulation, it is
considered part of the CESoPSN header.
The sequence number is used to provide the common PW sequencing
function, as well as detection of lost packets. It MUST be generated
in accordance with the rules defined in Section 5.1 of [RFC3550] for
the RTP sequence number, i.e.:
o Its space is a 16-bit unsigned circular space
o Its initial value SHOULD be random (unpredictable)
o It MUST be incremented with each CESoPSN data packet sent in the
specific PW.
4.4. Usage of the RTP Header
Although CESoPSN MAY employ an RTP header when explicit transfer of
timing information is required, this is purely formal reuse of the
header format. RTP mechanisms, such as header extensions,
contributing source (CSRC) list, padding, RTP Control Protocol
(RTCP), RTP header compression, Secure RTP (SRTP), etc., are not
applicable to CESoPSN pseudowires.
When a fixed RTP header (see [RFC3550], Section 5.1) is used with
CESoPSN, its fields are used in the following way:
1. V (version) is always set to 2.
2. P (padding), X (header extension), CC (CSRC count), and M
(marker) are always set to 0.
3. PT (payload type) is used as following:
a) One PT value MUST be allocated from the range of dynamic
values (see [RTP-TYPES]) for each direction of the PW. The
same PT value MAY be reused for both directions of the PW and
also reused between different PWs.
b) The PE at the PW ingress MUST set the PT field in the RTP
header to the allocated value.
c) The PE at the PW egress MAY use the received value to detect
malformed packets.
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4. Sequence number in the RTP header MUST be equal to the sequence
number in the CESoPSN CW.
5. Timestamps are used for carrying timing information over the
network:
a) Their values are generated in accordance with the rules
established in [RFC3550].
b) Frequency of the clock used for generating timestamps MUST be
an integer multiple of 8 kHz. All implementations of CESoPSN
MUST support the 8 kHz clock. Other frequencies that are
integer multiples of 8 kHz MAY be used if both sides agree to
that.
c) Possible modes of timestamp generation are discussed below.
6. The SSRC (synchronization source) value in the RTP header MAY be
used for detection of misconnections.
The RTP header in CESoPSN can be used in conjunction with at least
the following modes of timestamp generation:
1. Absolute mode: the ingress PE sets timestamps using the clock
recovered from the incoming TDM circuit. As a consequence, the
timestamps are closely correlated with the sequence numbers. All
CESoPSN implementations MUST support this mode.
2. Differential mode: PE devices connected by the PW have access to
the same high-quality synchronization source, and this
synchronization source is used for timestamp generation. As a
consequence, the second derivative of the timestamp series
represents the difference between the common timing source and
the clock of the incoming TDM circuit. Support of this mode is
OPTIONAL.
5. CESoPSN Payload Layer5.1. Common Payload Format Considerations
All the services considered in this document are treated as sequences
of "basic structures" (see Section 3 above). The payload of a
CESoPSN packet always consists of a fixed number of octets filled,
octet by octet, with the data contained in the corresponding
consequent basic structures that preserve octet alignment between
these structures and the packet payload boundaries, in accordance
with the following rules:
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1. The order of the payload octets corresponds to their order on the
TDM AC.
2. Consecutive bits coming from the TDM AC fill each payload octet,
starting from its most significant bit to the least significant
one.
3. All the CESoPSN packets MUST carry the same amount of valid TDM
data in both directions of the PW. In other words, the time that
is required to fill a CESoPSN packet with the TDM data must be
constant. The PE devices terminating a CESoPSN PW MUST agree on
the number of TDM payload octets in the PW packets for both
directions of the PW at the time of the PW setup.
Notes:
1. CESoPSN packets MAY omit invalid TDM data in order to save the
PSN bandwidth. If the CESoPSN packet payload is omitted, the L
bit in the CESoPSN control word MUST be set.
2. CESoPSN PWs MAY carry CE signaling information either in separate
packets or appended to packets carrying valid TDM data. If
signaling information and valid TDM data are carried in the same
CESoPSN packet, the amount of the former does not affect the
amount of the latter.
5.2. Basic NxDS0 Services
As mentioned above, the basic structure preserved across the PSN for
this service consists of N octets filled with the data of the
corresponding NxDS0 channels belonging to the same frame of the
originating trunk(s), and the service generates 8000 such structures
per second.
CESoPSN MUST use alignment of the basic structures with the packet
payload boundaries in order to carry the structures across the PSN.
This means that:
1. The amount of TDM data in a CESoPSN packet MUST be an integer
multiple of the basic structure size
2. The first structure in the packet MUST start immediately at the
beginning of the packet payload.
The resulting payload format is shown in Figure 3 below.
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o For 2 <=N <= 4: 4 millisecond (with the corresponding packet
payload size of 32*N bytes)
o For N >= 5: 1 millisecond (with the corresponding packet payload
size of 8*N octets).
Support of 5 ms packetization latency for N = 1 is RECOMMENDED.
Usage of any other packetization latency (packet payload size) that
is compatible with the restrictions described above is OPTIONAL.
5.3. Extending Basic NxDS0 Services with CE Application Signaling
Implementations that have chosen to extend the basic NxDS0 service to
support CE application state signaling carry-encoded CE application
state signals in separate signaling packets.
The format of the CESoPSN signaling packets over both IPv4/IPv6 and
MPLS PSNs for the case when the CE maintains a separate application
state per DS0 channel (e.g., CAS for the telephony applications) is
shown in Figures 4a and 4b below, respectively.
Signaling packets SHOULD be carried in a separate dedicated PW.
However, implementations MAY carry them in the same PW as the TDM
data packets for the basic NxDS0 service. The methods of "pairing"
the PWs carrying TDM data and signaling packets for the same extended
NxDS0 service are out of scope of this document.
Regardless of the way signaling packets are carried across the PSN,
the following rules apply:
1. The CESoPSN signaling packets MUST:
a) Use their own sequence numbers in the control word
b) Set the flags in the control word like following:
i) L = 0
ii) M = '11'
iii) R = 0
2. If an RTP header is used in the data packets, it MUST be also
used in the signaling packets with the following restrictions:
a) An additional RTP payload type (from the range of dynamically
allocated types) MUST be allocated for the signaling packets.
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b) In addition, the signaling packets MUST use their own SSRC
value.
The protocol used to assure reliable delivery of signaling packets is
discussed in Appendix A.
Encoding of CE application state for telephony applications using CAS
follows [RFC2833] (which has since been obsoleted by [RFC4733] and
[RFC4734], but they do not affect the relevant text).
Encoding of CE application state for telephony application using CCS
will be considered in a separate document.
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RFC 5086 TDM Circuit Emulation Service over PSN December 20075.4. Trunk-Specific NxDS0 Services with CAS
The structure preserved by CESoPSN for this group of services is the
trunk multiframe sub-divided into the trunk frames, and signaling
information is carried appended to the TDM data using the signaling
substructures defined in [ATM-CES]. These substructures comprise N
consecutive nibbles, so that the i-th nibble carries CAS bits for the
i-th DS0 channel, and are padded with a dummy nibble for odd values
of N.
CESoPSN implementations supporting trunk-specific NxDS0 services with
CAS MUST NOT carry more TDM data per packet than is contained in a
single trunk multiframe.
All CESoPSN implementations supporting trunk-specific NxDS0 with CAS
MUST support the default mode, where a single CESoPSN packet carries
exactly the amount of TDM data contained in exactly one trunk
multiframe and appended with the signaling sub-structure. The TDM
data is aligned with the packet payload. In this case:
1. Packetization latency is:
a) 2 milliseconds for E1 NxDS0
b) 3 milliseconds for T1 NxDS0
2. The packet payload size is:
a) 16*N + floor((N+1)/2) for E1-NxDS0
b) 24*N + floor((N+1)/2) for T1/ESF-NxDS0 and T1/SF- NxDS0
3. The packet payload format coincides with the multiframe structure
described in [ATM-CES] (Section 2.3.1.2).
In order to provide lower packetization latency, CESoPSN
implementations for trunk-specific NxDS0 with CAS SHOULD support
fragmentation of multiframe structures between multiple CESoPSN
packets. In this case:
1. The FRG bits MUST be used to indicate first, intermediate, and
last fragment of a multiframe as described in [RFC4623].
2. The amount of the TDM data per CESoPSN packet must be constant.
3. Each multiframe fragment MUST comprise an integer multiple of the
trunk frames.
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Notes:
1. In case of T1-NxDS0 with CAS, the signaling bits are carried in
the TDM data, as well as in the signaling substructure. However,
the receiver MUST use the CAS bits as carried in the signaling
substructures.
2. In case of trunk-specific NxDS0 with CAS originating in a T1-SF
trunk, each nibble of the signaling substructure contains A and B
bits from two consecutive trunk multiframes as described in
[ATM-CES].
6. CESoPSN Operation6.1. Common Considerations
Edge-to-edge emulation of a TDM service using CESoPSN is only
possible when the two PW attachment circuits are of the same type
(basic NxDS0 or one of the trunk-specific NxDS0 with CAS) and bit
rate. The service type and bit rate are exchanged at PW setup as
described in [RFC4447].
6.2. IWF Operation6.2.1. PSN-Bound Direction
Once the PW is set up, the PSN-bound CESoPSN IWF operates as follows:
TDM data is packetized using the configured number of payload bytes
per packet.
Sequence numbers, flags, and timestamps (if the RTP header is used)
are inserted in the CESoPSN headers and, for trunk-specific NxDS0
with CAS, signaling substructures are appended to the packets
carrying the last fragment of a multiframe.
CESoPSN, multiplexing layer, and PSN headers are prepended to the
packetized service data.
The resulting packets are transmitted over the PSN.
6.2.2. CE-Bound Direction
The CE-bound CESoPSN IWF SHOULD include a jitter buffer where payload
of the received CESoPSN packets is stored prior to play-out to the
local TDM attachment circuit. The size of this buffer SHOULD be
locally configurable to allow accommodation to the PSN-specific
packet delay variation.
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The CE-bound CESoPSN IWF MUST detect lost and misordered packets. It
SHOULD use the sequence number in the control word for these purposes
but, if the RTP header is used, the RTP sequence number MAY be used
instead.
The CE-bound CESoPSN IWF MAY reorder misordered packets. Misordered
packets that cannot be reordered MUST be discarded and treated as
lost.
The payload of the received CESoPSN data packets marked with the L
bit set SHOULD be replaced by the equivalent amount of some locally
configured "idle" bit pattern even if it has not been omitted. In
addition, the CE-bound CESoPSN IWF will be locally configured to
command its local NSP to perform one of the following actions:
o None (MUST be supported by all the implementations)
o Transmit the AIS pattern towards the local CE on the E1 or T1
trunk carrying the local attachment circuit (support of this
action is RECOMMENDED)
o Send the "Channel Idle" signal to the local CE for all the DS0
channels comprising the local attachment circuit (support of this
action is OPTIONAL).
If the data packets received are marked with L bit cleared and M bits
set to '10' or with R bit set, the CE-bound CESoPSN IWF will be
locally configured to command its local NSP to perform one of the
following actions:
o None (MUST be supported by all the implementations)
o Transmit the RAI pattern towards the local CE on the E1 or T1
trunk carrying the local attachment circuit (support of this
action is RECOMMENDED)
o Send the "Channel Idle" signal to the local CE for all the DS0
channels comprising the local attachment circuit (support of this
action is OPTIONAL and requires also that the CE-bound CES IWF
replaces the actually received payload with the equivalent amount
of the locally configured "idle" bit pattern.
Notes:
1. If the pair of IWFs at the two ends of the PW have been
configured to force the TDM trunks carrying their ACs to transmit
AIS upon reception of data packets with the L bit set and to
transmit RAI upon reception of data packets with the R bit set,
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RFC 5086 TDM Circuit Emulation Service over PSN December 2007
or with the L bit cleared and M bits set to '10', this PW
provides a bandwidth-saving emulation of a fractional E1 or T1
service between the pair of CE devices.
2. If the pair of IWFs at the two ends of the PW have been
configured to signal "Channel Idle" CE application state to its
local CE upon reception of packets marked with L bit set, R bit
set, or (L,M) set to '010', and to replace the actually received
payload with the locally configured "idle" bit pattern, the
resulting PW will comply with the requirements for Downstream
Trunk conditioning as defined in [TR-NWT-170].
3. Usage of bits R, L, and M described above additionally provides
the tools for "single-ended" management of the CESoPSN
pseudowires with ability to distinguish between the problems in
the PSN and in the TDM attachment circuits.
The payload of each lost CESoPSN data packet MUST be replaced with
the equivalent amount of the replacement data. The contents of the
replacement data are implementation-specific and MAY be locally
configurable. By default, all CESoPSN implementations MUST support
generation of the locally configurable "idle" pattern as the
replacement data.
Before a PW has been set up and after a PW has been torn down, the
IWF MUST play out the locally configurable "idle" pattern to its TDM
attachment circuit.
Once the PW has been set up, the CE-bound IWF begins to receive
CESoPSN packets and to store their payload in the jitter buffer, but
continues to play out the locally configurable "idle" pattern to its
TDM attachment circuit. This intermediate state persists until a
pre-configured amount of TDM data (usually half of the jitter buffer)
has been received in consecutive CESoPSN packets, or until a pre-
configured intermediate state timer expires.
Once the pre-configured amount of the TDM data has been received, the
CE-bound CESoPSN IWF enters its normal operation state, where it
continues to receive CESoPSN packets and store their payload in the
jitter buffer while playing out the contents of the jitter buffer in
accordance with the required clock. In this state, the CE-bound IWF
performs clock recovery, MAY monitor PW defects, and MAY collect PW
performance-monitoring data.
If the CE-bound CESoPSN IWF detects loss of a pre-configured number
of consecutive packets, or if the intermediate state timer expires
before the required amount of TDM data has been received, it enters
its packet loss state. While in this state:
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RFC 5086 TDM Circuit Emulation Service over PSN December 2007
o The locally configurable "idle" pattern SHOULD be played out to
the TDM attachment circuit.
o The local PSN-bound CESoPSN IWF SHOULD mark every packet it
transmits with the R bit set.
The CE-bound CESoPSN IWF leaves this state and transits to the normal
one once a pre-configured number of consecutive CESoPSN packets have
been received.
6.3. CESoPSN Defects
In addition to the packet loss state of the CE-bound CESoPSN IWF
defined above, it MAY detect the following defects:
o Stray packets
o Malformed packets
o Excessive packet loss rate
o Buffer overrun
o Remote packet loss.
Corresponding to each defect is a defect state of the IWF, a
detection criterion that triggers transition from the normal
operation state to the appropriate defect state, and an alarm that
MAY be reported to the management system and, thereafter, cleared.
Alarms are only reported when the defect state persists for a pre-
configured amount of time (typically 2.5 seconds) and MUST be cleared
after the corresponding defect is undetected for a second pre-
configured amount of time (typically 10 seconds). The trigger and
release times for the various alarms may be independent.
Stray packets MAY be detected by the PSN and multiplexing layers.
When RTP is used, the SSRC field in the RTP header MAY be used for
this purpose as well. Stray packets MUST be discarded by the CE-
bound IWF, and their detection MUST NOT affect mechanisms for
detection of packet loss.
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Malformed packets MAY be detected by mismatch between the expected
packet size (taking the value of the L bit into account) and the
actual packet size inferred from the PSN and multiplexing layers.
When RTP is used, lack of correspondence between the PT value and
that allocated for this direction of the PW MAY also be used for this
purpose. Other methods of detecting malformed packets are
implementation-specific. Malformed in-order packets MUST be
discarded by the CE-bound IWF and replacement data generated as for
lost packets.
Excessive packet loss rate is detected by computing the average
packet Loss rate over a configurable amount of times and comparing it
with a pre-configured threshold.
Buffer overrun is detected in the normal operation state when the
jitter buffer of the CE-bound IWF cannot accommodate newly arrived
CESoPSN packets.
Remote packet loss is indicated by reception of packets with their R
bit set.
6.4. CESoPSN PW Performance Monitoring
Performance monitoring (PM) parameters are routinely collected for
TDM services and provide an important maintenance mechanism in TDM
networks. Ability to collect compatible PM parameters for CESoPSN
PWs enhances their maintenance capabilities.
Collection of the CESoPSN PW performance monitoring parameters is
OPTIONAL and, if implemented, is only performed after the CE-bound
IWF has exited its intermediate state.
CESoPSN defines error events, errored blocks, and defects as follows:
o A CESoPSN error event is defined as insertion of a single
replacement packet into the jitter buffer (replacement of payload
of CESoPSN packets with the L bit set is not considered as
insertion of a replacement packet).
o A CESoPSN errored data block is defined as a block of data played
out to the TDM attachment circuit and of size defined in
accordance with the [G.826] rules for the corresponding TDM
service that has experienced at least one CESoPSN error event.
o A CESoPSN defect is defined as the packet loss state of the CE-
bound CESoPSN IWF.
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RFC 5086 TDM Circuit Emulation Service over PSN December 2007
The CESoPSN PW PM parameters (Errored, Severely Errored, and
Unavailable Seconds) are derived from these definitions, in
accordance with [G.826].
7. QoS Issues
If the PSN providing connectivity between PE devices is Diffserv-
enabled and provides a per-domain behavior (PDB) [RFC3086] that
guarantees low-jitter and low-loss, the CESoPSN PW SHOULD use this
PDB in compliance with the admission and allocation rules the PSN has
put in place for that PDB (e.g., marking packets as directed by the
PSN).
8. Congestion Control
As explained in [RFC3985], the PSN carrying the PW may be subject to
congestion. CESoPSN PWs represent inelastic, constant bit rate (CBR)
flows and cannot respond to congestion in a TCP-friendly manner
prescribed by [RFC2914], although the percentage of total bandwidth
they consume remains constant.
Unless appropriate precautions are taken, undiminished demand of
bandwidth by CESoPSN PWs can contribute to network congestion that
may impact network control protocols.
Whenever possible, CESoPSN PWs SHOULD be carried across traffic-
engineered PSNs that provide either bandwidth reservation and
admission control or forwarding prioritization and boundary traffic
conditioning mechanisms. IntServ-enabled domains supporting
Guaranteed Service (GS) [RFC2212] and Diffserv-enabled domains
[RFC2475] supporting Expedited Forwarding (EF) [RFC3246] provide
examples of such PSNs. Such mechanisms will negate, to some degree,
the effect of the CESoPSN PWs on the neighboring streams. In order
to facilitate boundary traffic conditioning of CESoPSN traffic over
IP PSNs, the CESoPSN IP packets SHOULD NOT use the Diffserv Code
Point (DSCP) value reserved for the Default PHB [RFC2474].
If CESoPSN PWs run over a PSN providing best-effort service, they
SHOULD monitor packet loss in order to detect "severe congestion".
If such a condition is detected, a CESoPSN PW SHOULD shut down
bidirectionally for some period of time as described in Section 6.5
of [RFC3985].
Note that:
1. The CESoPSN IWF can inherently provide packet loss measurement,
since the expected rate of arrival of CESoPSN packets is fixed
and known
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RFC 5086 TDM Circuit Emulation Service over PSN December 2007
2. The results of the CESoPSN packet loss measurement may not be a
reliable indication of presence or absence of severe congestion
if the PSN provides enhanced delivery, e.g.,:
a) If CESoPSN traffic takes precedence over non-CESoPSN traffic,
severe congestion can develop without significant CESoPSN
packet loss.
b) If non-CESoPSN traffic takes precedence over CESoPSN traffic,
CESoPSN may experience substantial packet loss due to a
short-term burst of high-priority traffic.
3. The TDM services emulated by the CESoPSN PWs have high
availability objectives (see [G.826]) that MUST be taken into
account when deciding on temporary shutdown of CESoPSN PWs.
This specification does not define the exact criteria for detecting
"severe congestion" using the CESoPSN packet loss rate, or the
specific methods for bidirectional shutdown that the CESoPSN PWs
(when such severe congestion has been detected) and their consequent
restart after a suitable delay. This is left for further study.
However, the following considerations may be used as guidelines for
implementing the CESoPSN severe congestion shutdown mechanism:
1. CESoPSN Performance Monitoring techniques (see Section 6.4)
provide entry and exit criteria for the CESoPSN PW "Unavailable"
state that make it closely correlated with the "Unavailable"
state of the emulated TDM circuit as specified in [G.826]. Using
the same criteria for "severe congestion" detection may decrease
the risk of shutting down the CESoPSN PW while the emulated TDM
circuit is still considered available by the CE.
2. If the CESoPSN PW has been set up using either PWE3 control
protocol [RFC4447] or L2TPv3 [RFC3931], the regular PW teardown
procedures of these protocols SHOULD be used.
3. If one of the CESoPSN PW end points stops transmission of packets
for a sufficiently long period, its peer (observing 100% packet
loss) will necessarily detect "severe congestion" and also stop
transmission, thus achieving bidirectional PW shutdown.
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RFC 5086 TDM Circuit Emulation Service over PSN December 20079. Security Considerations
CESoPSN does not enhance or detract from the security performance of
the underlying PSN; rather, it relies upon the PSN mechanisms for
encryption, integrity, and authentication whenever required.
CESoPSN PWs share susceptibility to a number of pseudowire-layer
attacks, and will use whatever mechanisms for confidentiality,
integrity, and authentication that are developed for general PWs.
These methods are beyond the scope of this document.
Although CESoPSN PWs MAY employ an RTP header when explicit transfer
of timing information is required, it is not possible to use SRTP
(see [RFC3711]) mechanisms as a substitute for PW layer security.
Misconnection detection capabilities of CESoPSN increase its
resilience to misconfiguration and some types of DoS attacks.
Random initialization of sequence numbers, in both the control word
and the optional RTP header, makes known-plaintext attacks on
encrypted CESoPSN PWs more difficult. Encryption of PWs is beyond
the scope of this document.
10. IANA Considerations
Allocation of PW Types for the corresponding CESoPSN PWs is defined
in [RFC4446].
11. Applicability Statement
CESoPSN is an encapsulation layer intended for carrying NxDS0
services with or without CAS over PSN.
CESoPSN allows emulation of certain end-to-end delay properties of
TDM networks. In particular, the end-to-end delay of a TDM circuit
emulated by a CESoPSN PW does not depend upon the bit rate of the
service.
CESoPSN fully complies with the principle of minimal intervention,
minimizing overhead, and computational power required for
encapsulation.
CESoPSN can be used in conjunction with various clock recovery
techniques and does not presume availability of a global synchronous
clock at the ends of a PW. However, if the global synchronous clock
is available at both ends of a CESoPSN PW, using RTP and differential
mode of timestamp generation improves the quality of the recovered
clock.
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RFC 5086 TDM Circuit Emulation Service over PSN December 2007
CESoPSN allows carrying CE application state signaling that requires
synchronization with data in-band in separate signaling packets. A
special combination of flags in the CESoPSN control word is used to
distinguish between data and signaling packets, while the Timestamp
field in the RTP headers is used for synchronization. This makes
CESoPSN extendable to support different types of CE signaling without
affecting the data path in the PE devices.
CESoPSN also allows emulation of NxDS0 services with CAS carrying the
signaling information appended to (some of) the packets carrying TDM
data.
CESoPSN allows the PSN bandwidth conservation by carrying only AIS
and/or Idle Code indications instead of data.
CESoPSN allows deployment of bandwidth-saving Fractional point-to-
point E1/T1 applications. These applications can be described as the
following:
o The pair of CE devices operates as if it was connected by an
emulated E1 or T1 circuit. In particular, it reacts to AIS and
RAI states of its local ACs in the standard way.
o The PSN carries only an NxDS0 service, where N is the number of
actually used timeslots in the circuit connecting the pair of CE
devices, thus saving the bandwidth.
Being a constant bit rate (CBR) service, CESoPSN cannot provide TCP-
friendly behavior under network congestion. If the service
encounters congestion, it SHOULD be temporarily shut down.
CESoPSN allows collection of TDM-like faults and performance
monitoring parameters; hence, emulating 'classic' carrier services of
TDM circuits (e.g., SONET/SDH). Similarity with these services is
increased by the CESoPSN ability to carry 'far end error'
indications.
CESoPSN provides for a carrier-independent ability to detect
misconnections and malformed packets. This feature increases
resilience of the emulated service to misconfiguration and DoS
attacks.
CESoPSN provides for detection of lost packets and allows using
various techniques for generation of "replacement packets".
CESoPSN carries indications of outages of incoming attachment circuit
across the PSN, thus, providing for effective fault isolation.
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RFC 5086 TDM Circuit Emulation Service over PSN December 2007
Faithfulness of a CESoPSN PW may be increased if the carrying PSN is
Diffserv-enabled and implements a PDB that guarantees low loss and
low jitter.
CESoPSN does not provide any mechanisms for protection against PSN
outages. As a consequence, resilience of the emulated service to
such outages is defined by the PSN behavior. On the other hand:
o The jitter buffer and packets' reordering mechanisms associated
with CESoPSN increase resilience of the emulated service to fast
PSN re-convergence events
o Remote indication of lost packets is carried backward across the
PSN from the receiver (that has detected loss of packets) to
transmitter. Such an indication MAY be used as a trigger for
activation of proprietary, service-specific protection mechanisms.
Security of TDM services provided by CESoPSN across a shared PSN may
be below the level of security traditionally associated with TDM
services carried across TDM networks.
12. Acknowledgements
Akiva Sadovski has been an active participant of the team that co-
authored early versions of this document.
We express deep gratitude to Stephen Casner, who reviewed an early
version of this document in detail, corrected some serious errors,
and provided many valuable inputs.
The present version of the text of the QoS section has been suggested
by Kathleen Nichols.
We thank Maximilian Riegel, Sim Narasimha, Tom Johnson, Ron Cohen,
and Yaron Raz for valuable feedback.
We thank Alik Shimelmits for many fruitful discussions.
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RFC 5086 TDM Circuit Emulation Service over PSN December 2007Appendix A. A Common CE Application State Signaling Mechanism
Format of the CESoPSN signaling packets is discussed in Section 5.3
above.
The sequence number in the CESoPSN control word for the signaling
packets is generated according to the same rules as for the TDM data
packets.
If the RTP header is used in the CESoPSN signaling packets, the
timestamp in this header represents the time when the CE application
state has been collected.
Signaling packets are generated by the ingress PE, in accordance with
the following logic (adapted from [RFC2833]):
1. The CESoPSN signaling packet with the same information (including
the timestamp in the case RTP header is used) is sent 3 times at
an interval of 5 ms under one of the following conditions:
a) The CESoPSN PW has been set up
b) A change in the CE application state has been detected. If
another change of the CE application state has been detected
during the 10 ms period (i.e., before all 3 signaling packets
reporting the previous change have been sent), this process is
re-started, i.e.:
i) The unsent signaling packet(s) with the previous CE
application state are discarded
ii) Triple send of packets with the new CE application state
begins.
c) Loss of packets defect has been cleared
d) Remote Loss of Packets indication has been cleared (after
previously being set)
2. Otherwise, the CESoPSN signaling packet with the current CE
application state information is sent every 5 seconds.
These rules allow fast probabilistic recovery after loss of a single
signaling packet, as well as deterministic (but possibly slow)
recovery following PW setup and PSN outages.
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RFC 5086 TDM Circuit Emulation Service over PSN December 2007Appendix B. Reference PE Architecture for Emulation of NxDS0 Services
Structured TDM services do not exist as physical circuits. They are
always carried within appropriate physical attachment circuits (AC),
and the PE providing their emulation always includes a Native Service
Processing Block (NSP), commonly referred to as Framer. As a
consequence, the architecture of a PE device providing edge-to-edge
emulation for these services includes the Framer and Forwarder
blocks.
In case of NxDS0 services (the only type of structured services
considered in this document), the AC is either an E1 or a T1 trunk,
and bundles of NxDS0 are cut out of it using one of the framing
methods described in [G.704].
In addition to detecting the FAS and imposing associated structure on
the "trunk" AC, E1, and T1, framers commonly support some additional
functionality, including:
1. Detection of special states of the incoming AC (e.g., AIS, OOF,
or RAI)
2. Forcing special states (e.g., AIS and RAI) on the outgoing AC
upon explicit request
3. Extraction and insertion of CE application signals that may
accompany specific DS0 channel(s).
The resulting PE architecture for NxDS0 services is shown in Figure
B.1 below. In this diagram:
1. In the PSN-bound direction:
a) The Framer:
i) Detects frame alignment signal (FAS) and splits the
incoming ACs into separate DS0 channels
ii) Detects special AC states
iii) If necessary, extracts CE application signals accompanying
each of the separate DS0 services
b) The Forwarder:
i) Creates one or more NxDS0 bundles
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RFC 5086 TDM Circuit Emulation Service over PSN December 2007
ii) Sends the data received in each such bundle to the PSN-
bound direction of a respective CESoPSN IWF instance
iii) If necessary, sends the current CE application state data
of the DS0 services in the bundle to the PSN-bound
direction of the respective CESoPSN IWF instance
iv) If necessary, sends the AC state indications to the PSN-
bound directions of all the CESoPSN instances associated
with the given AC
c) Each PSN-bound PW IWF instance encapsulates the received data,
application state signal, and the AC state into PW PDUs, and
sends the resulting packets to the PSN
2. In the CE-bound direction:
a) Each CE-bound instance of the CESoPSN IWF receives the PW PDUs
from the PSN, extracts the TDM data, AC state, and CE
application state signals, and sends them
b) The Forwarder sends the TDM data, application state signals
and, if necessary, a single command representing the desired
AC state, to the Framer
c) The Framer accepts all the data of one or more NxDS0 bundles
possibly accompanied by the associated CE application state,
and commands referring to the desired AC state, and generates
a single AC accordingly with correct FAS.
Notes: This model is asymmetric:
o AC state indication can be forwarded from the framer to multiple
instances of the CESoPSN IWF
o No more than one CESoPSN IWF instance should forward AC state-
affecting commands to the framer.
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RFC 5086 TDM Circuit Emulation Service over PSN December 2007
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